The present invention relates to the field of fungal biotechnology, more particularly to genetic engineering methods for the production of omega-3 polyunsaturated fatty acids (PUFAs) in fungal hosts selected from Rhodospordium and Rhodotorula genera.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the Bibliography.
Omega-3 fatty acids (also called ω-3 fatty acids or n-3 fatty acids) refer to alpha-linolenic acid (ALA) [(9Z,12Z,15Z)-9,12,15-Octadecatrienoic acid], EPA (eicosapentaenoic acid, or [(5Z,8Z,11Z,14Z,17Z)-5,8,11,14,17-Eicosapentaenoic acid]) and DHA [docosahexaenoic acid, or (4Z,7Z,10Z,13Z,16Z,19Z)-docosa-4,7,10,13,16,19-hexaenoic acid]. Common sources of animal omega-3 EPA and DHA fatty acids include fish oils, egg oil, squid oils and krill oil while some plant oils, such as oil from seabuckthorn seed and berry, algal cells, flax seed, Chia seed and hemp seed, contain high levels of ALA.
Linoleic acid [(9Z,12Z)-9,12-Octadecadienoic acid], gamma-linolenic acid (GLA, or all-cis-6,9,12-octadecatrienoic acid) and arachidonic acid [(5Z,8Z,11Z,14Z)-5,8,11,14-Eicosatetraenoic acid]) are omega-6 fatty acids. GLA is an omega-6 fatty acid that is found mostly in plant based oils such as borrage seed oil, evening primrose oil, and black currant seed oil.
Omega-3 fatty acids are vital for normal metabolism. Omega-3s are considered essential fatty acids, i.e., cannot be synthesized by the human body except that mammals have a limited ability, when the diet includes the shorter-chain omega-3 fatty acid ALA, to form the more important long-chain omega-3 fatty acids, EPA and then from EPA, the most crucial, DHA with even greater inefficiency. It is now accepted that omega-3 polyunsaturated fatty acids, especially EPA and DHA play important roles in a number of aspects of human health. However, over-fishing and concerns about pollution of the marine environment indicate a need to develop alternative, sustainable sources of very long chain polyunsaturated fatty acids (VLC-PUFAs) such as EPA and DHA [1]. Omega-6 fatty acids are considered essential fatty acids: They are necessary for human health. Along with omega-3 fatty acids, omega-6 fatty acids play a crucial role in brain function, as well as normal growth and development. Omega-3 fatty acids and omega-6 fatty acids help stimulate skin and hair growth, maintain bone health, regulate metabolism, and maintain the reproductive system [2]. Some preliminary clinical research suggests that GLA may be useful for Diabetic neuropathy, Rheumatoid arthritis, Allergies, Eczema, High blood pressure (Hypertension), Menopausal symptoms, etc. The ratio of dietary intake of omega-6 and omega-3 essential fatty acids is considered important for health in human [3].
A large number of oleaginous microorganisms have been reported to date. The oil they produce, often referred to as Single Cell Oil (SCO), is similar to those of plants and can be used for the production of biodiesel, food and industrial products [4-6]. SCO is now widely accepted in the market place and there is a growing awareness of the health benefits of PUFAs, such as γ-linolenic acid (GLA), arachidonic acid (ARA), DHA and EPA. ARA and DHA have also been used for fortification of infant formulae in many parts of the world. Fish oils are rich sources of DHA and EPA and a limited number of plant oilseeds are good sources of other PUFAs. Marine protists and dinoflagellates, such as species of Thraustochytrium, Schizochytrium and Crypthecodinium are the rich sources of DHA, whereas microalgae like Phaeodactylum and Monodus are good sources of EPA. Species of lower fungi Mortierella accumulate a high percentage of ARA in the lipid fraction [7].
While yeast Yarrowia lipolytica perhaps has enjoyed long history of research and development as a bioengineering host for SCO [8-12], Rhodosporidium toruloides (also known as Rhodotorula glutenis) has attracted increasing attention due to its ability to perform higher cell density fermentation at a fast growth rate, efficiently producing cell mass with an oil content of >67% (w/w dry cell mass) [13-16].
The Pucciniomycotina is a subphylum of fungi in the phylum of Basidiomycota [17]. It holds many species that have important industrial applications. For example, a number of species in the Rhodosporidium and Sporidiobolus genera, such as Rhodosporidium toruloides (also known as Rhodotorula gracilis, Rhodosporidium glutinis, Rhodotorula glutinis, Torula koishikawensis and Torula rubescens) and Sporobolomyces salmonicolor, are oil-rich single-cell yeasts capable of high density fermentation [6, 18]. These species hold great potential as a host for the production of long chain hydrocarbons, such as triacylglycerol (TAG, or fat), fatty acid esters (biodiesel), fatty alcohols, alcohols, lactones, terpenoids and vitamins [14, 19-21].
Rhodosporidium and Rhodotorula genomes are highly GC-rich, which has been found to profoundly influence genetic transformation and protein expression [22-24]. Metabolic engineering is an effective technique for improving production of metabolites in plants and microbes. In terms of bioengineering for omega-3 fatty acids, expressing various desaturases and elongases, both in plants and oleaginous yeast, are critical for the production of PUFAs [25]. GLA is synthesized from linoleic acid (LA; C18:2Δ9,12 cis) by Δ6-desaturase. The seed oil of safflower (Carthamus tinctorius) contains high LA and has been modified by transformation with Δ6-desaturases from Mortierella alpina and Saprolegnia diclina to achieve more than 50% (v/v) of GLA respectively [26].
ALA and GLA are both precursors for the production of longer chains omega-3 fatty acids, such as arachidonic acid (AA), EPA and docosahexaenoic acid (DHA) [7, 27]. Therefore, the ability to produce high levels of ALA and GLA at high volumetric productivity is crucial for the bioengineering of longer chain PUFAs in Rhodosporidium toruloides. Thus, there is a need to develop fungal species of the Rhodospordium and Rhodotorula genera that produce high levels of ALA and GLA that are then available in the fungal species for the production of longer chain PUFAs.